Field of the Invention
This invention relates to tubular heat exchangers, and in particular to turbulence-inducing devices positioned in the tubes of the tubular heat exchanger that minimize or prevent fouling caused by the heat transfer fluids and enhance or maintain the overall heat transfer coefficient over the operational life of the tubular members.
Description of Related Art
Heat exchangers are found in many industrial and commercial applications. In the design of heat transfer equipment, an important factor includes the footprint of the exchanger relative to the capacity of fluid that is to be heated or cooled (the “receiving fluid”), as well as the requisite flow of the heating or cooling fluid (the “transferring fluid”). The heat transfer coefficient between the transferring fluid and the receiving fluid should be maximized to achieve the smallest allowable footprint of the heat exchanger.
Another factor that must be considered in designing heat exchangers is the tendency of heating or cooling fluids to foul in the tubes through which they pass. One detrimental effect of fouling is a lowering of the heat transfer coefficient. The thermal conductivity of the fouling layer is less than that of the tube material, which increases the heat transfer resistance, reduces the efficacy of the heat exchanger, and increases the tube skin temperature. Another negative effect of fouling is that the formation of depositions on the interior surface of the tubes reduces their cross-sectional area, causing increased resistance to the fluid flow and an increase in the pressure drop across the unit.
In refinery and petrochemical plants, problems caused by tube fouling are very expensive to remedy. Capital expenditures are higher due to the increased size of the heat exchanger (e.g., selecting heat exchangers with 10-50% greater surface area to accommodate conventional fouling expectations), the associated increase in requisite area within the plant, the higher strength and size foundations, and the extra transport and installation costs. Furthermore, the cost of operating the unit is increased due to additional fuel, electricity or process steam requirements. In addition, production losses occur during planned and emergency plant shutdowns due to fouling and associated system failures.
Various attempts to minimize or prevent fouling problems have been advanced. One common prevention technique is to use a fouling factor in the design phase of a heat transfer unit that includes increasing the heat transfer surface area, either by increasing the number of tubes or the tube length. Such a fouling factor is considered a necessary aspect of heat exchanger design, based on acceptance of the fact that fouling is inevitable. In addition to the aforementioned costs associated with selecting a larger heat exchanger, an additional concern is that the excess surface area calculated with a fouling factor can result in start-up complications and actually encourage more fouling. That is, it is common that at start-up, sludge and dirt migrate into dead zones and low velocity locations. The effect of increasing the number of tubes is to decrease the fluid flow velocity, thereby increasing the likelihood of fouling. Similarly, increasing the tube length results in lower fluid pressure, also increasing the likelihood of fouling.
Other known attempts to mitigate fouling problems involve the use of in-line mechanical cleaning devices to remove fouling build-up inside the tubes. These devices, which generally require direct physical contact with the inner tube surface, have not been especially successful in preventing fouling.
Deflection insertions are also another general category of fouling prevention or mitigation devices. For instance, U.S. Pat. No. 1,015,831 to Pielock et al. discloses a device that is inserted in a pipe to deflect the central and peripheral flow of liquid. Fluid along the side walls is directed toward the center of the pipe, and fluid moving along the longitudinal center line is directed towards the side walls. The device is constructed as a ring installed on the pipe's inner surface having a diametrically disposed web or a plurality of webs that form an apex pointed against the direction of fluid flow. However, the device described in Pielock et al. is mainly intended to diffuse central flow in multiphase fluid for equal distribution. Furthermore, in the context of a heat exchanger's transferring tube, fouling will predictably occur at the interface of the Pielock device and the tube's inner surface.
U.S. Pat. No. 3,995,663 to Perry describes a ferrule for insertion at the inlet of a vertical shell-and-tube heat exchanger, including a flange and shoulder to seat upon the tube sheet, a bore and a cylindrical portion as an extension of the bore to facilitate formation of a solid column of liquid entering the tube. The ferrule also includes an outwardly extending connecting wall that distributes fluid towards the apex of a conical member. Fluid entering the bore is directed to the side walls due to the shape of the conical member. Apparently, the purpose of the device is to distribute liquid to the walls of the ferrule rather than to the tube walls to provide liquid in the form of a falling film on the inner surfaces of the vertical tubes for evaporation. Therefore, application of this structure is necessarily limited to vertical shell-and-tube heat exchangers.
U.S. Pat. No. 5,311,929 to Verret and U.S. Pat. No. 4,794,980 to Raisanaen both disclose air-to-air heat exchangers that include cone-shaped elements disposed in each tube along a central rod. The cones serve as deflectors to create turbulence in the gases flowing through the tube. The elements disclosed in Verret are attached using a twisted strip of material bent inside the tubes to provide contact with the tube's internal surface. The conical elements described in Raisanaen are open on the downstream end, thus allowing fouling and sludge accumulation inside the cone.
The above-described references each have drawbacks that render them unsuitable for minimizing or preventing fouling. Additional known attempts to prevent fouling rely upon inserts fixed to the inner wall of the tube. However, fouling will eventually accumulate at, and proximate to the attachment points, which hinders removal of the inserts and thus complicates cleaning the inner surface of the tube.
Therefore, it is an object of the present invention to provide an apparatus for use in the tubes of heat exchangers that eliminates or minimizes fouling of the interior surfaces of the tubes.
It is another object of the present invention to provide an apparatus for use in tubes of heat exchangers that maintains the heat transfer coefficient over the operational life of the tubes.
It is still another object of the present invention to provide an apparatus for use in the tubes of heat exchangers that permits the designer to utilize the minimum theoretical heat exchanger size or capacity for a given application.